Double-Slit Experiment
Interactive simulation of Young's famous double-slit experiment. Watch wavefronts pass through two slits and observe the interference pattern on the detection screen.
- Slit Separation (d): Distance between the two slits. Larger separation = more fringes
- Wavelength (λ): Light wavelength (380-700 nm visible). Shorter wavelength = narrower fringes
- Screen Distance (L): Distance from slits to detection screen. Farther = wider fringes
- Bright Fringes: Occur where waves constructively interfere (path difference = nλ)
- Dark Fringes: Occur where waves destructively interfere (path difference = (n+½)λ)
- Animation: Click Start to see wavefronts propagate through the slits
📚 Physics Background
🔬 Young's Double-Slit Experiment
In 1801, Thomas Young performed a groundbreaking experiment that demonstrated the wave nature of light. By passing light through two closely spaced slits, he observed an interference pattern on a screen—alternating bright and dark bands that could only be explained if light behaved as a wave.
This experiment was pivotal in establishing the wave theory of light and remains a cornerstone demonstration in quantum mechanics, where even single particles exhibit wave-like interference.
📐 Mathematical Formulation
The interference pattern depends on the path difference between waves from the two slits:
Path difference: Δ = d sin(θ)
For constructive interference (bright fringes):
d sin(θ) = nλ (n = 0, ±1, ±2, ...)
For destructive interference (dark fringes):
d sin(θ) = (n + ½)λ
Where:
- d is the slit separation
- λ is the wavelength of light
- θ is the angle from the central axis
- n is the order number (0 for central maximum)
📏 Fringe Spacing
For small angles (sin θ ≈ tan θ ≈ y/L), the fringe spacing is:
Δy = λL/d
This formula shows that fringe spacing increases with wavelength (λ) and screen distance (L), but decreases with slit separation (d). This is why you see wider fringes when using red light compared to blue light, or when moving the screen farther away.
🌊 Wave Interference
✅ Constructive Interference (Bright Fringes)
Condition: Path difference = nλ (integer number of wavelengths)
When waves arrive in phase, their amplitudes add together, creating maximum brightness. The central bright fringe (n=0) is always the brightest as both waves travel equal distances.
❌ Destructive Interference (Dark Fringes)
Condition: Path difference = (n+½)λ (half-integer wavelengths)
When waves arrive 180° out of phase, their amplitudes cancel, creating darkness. These dark regions appear between the bright fringes.
🔮 Quantum Implications
The double-slit experiment is central to quantum mechanics. When performed with single photons or electrons, the interference pattern still emerges over time—even though each particle passes through one slit at a time. This demonstrates wave-particle duality.
Even more mysteriously, if we try to detect which slit a particle passes through, the interference pattern disappears! This "observer effect" highlights fundamental aspects of quantum measurement.
🔬 Applications
- Wavelength Measurement: The pattern can precisely determine light wavelength
- Coherence Testing: Interference visibility indicates source coherence
- Holography: Uses interference principles to create 3D images
- Gravitational Wave Detection: LIGO uses interference to detect tiny space-time ripples
- Thin Film Coatings: Anti-reflective coatings exploit interference